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What Gravitational Waves Can Say About Dark Matter

symmetrymagazine.org

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Re: What Gravitational Waves Can Say About Dark Matter

#11

Post by canjobear »

mirimir wrote: post #5

TFA doesn't mention recent evidence that primordial black holes are rare. It also doesn't address issues around condensation of dark matter. That is, two dark-matter objects can't collide, because they'll just pass through each other.

Dark matter objects interact with regular matter only via gravity. But the article seems to suggest that it might interact with itself by some other force, which may allow for "collision":

"Dark matter seems to interact with normal matter only through gravity, but, based on the way known particles interact, theorists think it’s possible that dark matter might also interact with itself. "

Re: What Gravitational Waves Can Say About Dark Matter

#12

Post by wwarner »

Tangential, and possibly revealing ignorance here. I definitely don't get why clouds of dark matter surrounding galaxies don't fall into the black hole at the center. Dark matter is there to explain why the outer stars of a galaxy rotate faster than expected, but why is dark matter not distributed roughly in the same density distribution as visible matter?

Re: What Gravitational Waves Can Say About Dark Matter

#13

Post by InclinedPlane »

wwarner wrote: post #12

Tangential, and possibly revealing ignorance here. I definitely don't get why clouds of dark matter surrounding galaxies don't fall into the black hole at the center. Dark matter is there to explain why the outer stars of a galaxy rotate faster than expected, but why is dark matter not distributed roughly in the same density distribution as visible matter?

They do. But there's nothing special about the gravity of a black hole versus the gravity of "ordinary matter". The vast majority of the matter in a galaxy is in orbit and won't get anywhere near close enough to the central black hole to fall into it. This is true of stars, planets, gas, and dust just as it's true of dark matter.

Re: What Gravitational Waves Can Say About Dark Matter

#14

Post by wrycoder »

wwarner wrote: post #12

Tangential, and possibly revealing ignorance here. I definitely don't get why clouds of dark matter surrounding galaxies don't fall into the black hole at the center. Dark matter is there to explain why the outer stars of a galaxy rotate faster than expected, but why is dark matter not distributed roughly in the same density distribution as visible matter?

It is presumed that the dark matter also rotates about the galactic center. It can’t fall in, any more than the stars in the galaxy can, because it collectively can’t dump its angular momentum. However, there is controversy whether dark matter can dissipate momentum that is parallel to the rotational axis either by self interaction or by interaction with ordinary matter. If it can, then the distribution of dark matter could resemble a disk, as opposed to a spherical distribution.

Lisa Randall has written a provocative book about this which will answer your question in depth: Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe.

Re: What Gravitational Waves Can Say About Dark Matter

#15

Post by scottie_m »

wwarner wrote: post #12

Tangential, and possibly revealing ignorance here. I definitely don't get why clouds of dark matter surrounding galaxies don't fall into the black hole at the center. Dark matter is there to explain why the outer stars of a galaxy rotate faster than expected, but why is dark matter not distributed roughly in the same density distribution as visible matter?

As others have pointed out, if they’re in a stable orbit they won’t fall in (at least not for a very long time), because unlike luminous matter it won’t experience any force other than gravity. If you imagine a dust cloud of luminous matter around a black hole, it will tend to experience frictional heating the closer it gets, there is the chance of a collision or radioactive decay, and other forces acting to draw it in or send it far away. Dark matter won’t do that, it just couples to gravity. Our usual intuition about how a halo of matter behaves has a lot to do with interactions other than gravity. Clumping for example, aggregation and accretion pretty much work because of interactions other than gravity, until a body becomes massive enough.

Re: What Gravitational Waves Can Say About Dark Matter

#16

Post by scottie_m »

FakeComments wrote: post #8
casual_slacker wrote: post #2

Perhaps tangential, but is there a frequency of a gravity wave high enough to "cusp" due to one of the quantization limits?

In even the classic theory, when the Swartzchild radius of the gravitational wavelet exceeds its width, wouldn’t it naturally form a black hole and end up a cusp? Like a gravity Kugelblitz.

I think by definition it wouldn’t form a black hole, it would be a black hole. Gravitational waves are propagating disturbances through spacetime, so the kind of wave you’re describing would begin as a singularity. I don’t think (but am not sure) that the math allows for the emission of such a thing. It sounds non-physical, and I’d suspect that if you do the math you’d discover that you’d need to have giant black holes merging to generate such a wave, or FTL. In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form.

Re: What Gravitational Waves Can Say About Dark Matter

#17

Post by wallace_f »

scottie_m wrote: post #16
FakeComments wrote: post #8

Earlier quoted context omitted.

In even the classic theory, when the Swartzchild radius of the gravitational wavelet exceeds its width, wouldn’t it naturally form a black hole and end up a cusp? Like a gravity Kugelblitz.

I think by definition it wouldn’t form a black hole, it would be a black hole. Gravitational waves are propagating disturbances through spacetime, so the kind of wave you’re describing would begin as a singularity. I don’t think (but am not sure) that the math allows for the emission of such a thing. It sounds non-physical, and I’d suspect that if you do the math you’d discover that you’d need to have giant black hol…

>In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form.

Sorry if this is a bit naive and tangential, but I've always stumbled at the thought of how does gravity-information about the interior of a black hole propagate out of the event horizon? ...Gravitons/gravity waves travel at, c?

Re: What Gravitational Waves Can Say About Dark Matter

#18

Post by scottie_m »

wallace_f wrote: post #17
scottie_m wrote: post #16

Earlier quoted context omitted.

I think by definition it wouldn’t form a black hole, it would be a black hole. Gravitational waves are propagating disturbances through spacetime, so the kind of wave you’re describing would begin as a singularity. I don’t think (but am not sure) that the math allows for the emission of such a thing. It sounds non-physical, and I’d suspect that if you do the math you’d discover that you’d need to have giant black hol…

>In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form. Sorry if this is a bit naive and tangential, but I've always stumbled at the thought of how does gravity-information about the interior of a black hole propagate out of the event horizon? ...Gravitons/gravity waves travel at, c?

That’s a bit of a tricky question, because it’s math-heavy. The best way to describe it is to think of the event horizon as the black hole, and forget that there is even an interior. The black hole can be fully described by the conditions at the event horizon after all, and everything else is cut off from the surrounding universe completely. In that sense there is no propagation from the interior at all, which is good because if information could escape then theories describing black holes would be broken.

Instead the black hole has mass, charge, and momentum (three kinds of momentum actually, but that’s not important). Whatever is going on beyond the event horizon, whatever that might be, has no effect that anyone can detect. Matter is accreted “onto” the event horizon which then expands in proportion to the mass of the volume of the hole. Maybe it’s destroyed beyond that point, or maybe it goes to another universe, but we can never know. The event horizon can also shrink if the surroundings are sufficiently cold (really really really cold) and the horizon is sufficiently hot.

Still, all of this is surface phenomena, like dropping a bowling ball into a tub of water. The water only “knows” about the surface of the ball, which which gets properties from the whole ball without exposing the center. A bowling ball in water creates waves, but the interior isn’t interacting with the water anymore than the black hole interior interacts with space (assuming an idealized perfectly rigid bowling ball). In the same way gravity waves or fractions would be a function of how the space just beyond the event horizon is warped.

Does that help?

Re: What Gravitational Waves Can Say About Dark Matter

#19

Post by andreareina »

Dark matter seems to interact with normal matter only through gravity, but, based on the way known particles interact, theorists think it’s possible that dark matter might also interact with itself.

If dark matter were to interact with itself (as regular matter does), wouldn't we expect it to clump together and form the same structures as regular matter does, especially given that there's supposed to be much more dark matter to interact together than the regular stuff? AIUI one of the defining features of dark matter is that it doesn't clump together.

Re: What Gravitational Waves Can Say About Dark Matter

#20

Post by nuccy »

andreareina wrote: post #19

Dark matter seems to interact with normal matter only through gravity, but, based on the way known particles interact, theorists think it’s possible that dark matter might also interact with itself. If dark matter were to interact with itself (as regular matter does), wouldn't we expect it to clump together and form the same structures as regular matter does, especially given that there's supposed to be much more dar…

Actually dark matter clumps, e.g. so called dark matter halo of a galaxy[1]. It is seen from rotational curves of galaxies and can be also checked with with strong and weak lensing effects [2].

[1] https://en.wikipedia.org/wiki/Dark_matter_halo

[2] https://en.wikipedia.org/wiki/Gravitational_lens

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